Resistance welding device comprising a power source arranged on a welding gun

ABSTRACT

A resistance welding device includes a power source in order to provide a welding current for welding workpieces, a welding gun with two gun arms having one electrode each in order to impress the welding current into the workpieces. The power source includes a heavy-current transformer with at least one primary winding and at least one secondary winding with center tapping, a synchronous rectifier connected with the secondary winding and including circuit elements, and a circuit for actuating the circuit elements of the synchronous rectifier. For reduction of losses and improvement of efficiency, the power source is arranged at the welding gun and the power source includes at least four contacts forming a multi-point contacting, wherein two first contacts of one polarity are connected to one gun arm and two additional contacts of opposite polarity are connected to the other gun arm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of PCT/AT2012/000275 filed onOct. 31, 2012, which claims priority under 35 U.S.C. § 119 of AustrianApplication No. A 1599/2011 filed on Oct. 31, 2011, the disclosure ofwhich is incorporated by reference. The international application underPCT article 21(2) was not published in English.

BACKGROUND OF THE INVENTION Field of Invention

The invention relates to a resistance welding device comprising a powersource in order to provide a welding current for welding workpieces, awelding gun with two gun arms comprising one electrode each in order toimpress the welding current into the workpieces, wherein the powersource comprises a heavy-current transformer with at least one primarywinding and at least one secondary winding with center tapping, asynchronous rectifier connected with the at least one secondary windingof the heavy-current transformer and comprising circuit elements, and acircuit for actuating the circuit elements of the synchronous rectifier.

The present invention relates to resistance welding devices, inparticular spot welding devices, in which especially heavy directcurrents in the order of some kA occur. Other devices, in which suchheavy direct currents are employed, are also covered in the subjectmatter of the present patent application. Examples of such devices arebattery chargers, particle accelerators, equipment for electroplating orthe like. WO 2007/041729 A1, for example, describes a battery chargerand a current transformer for producing a correspondingly heavy directcurrent.

Prior Art

In resistance welding devices the required heavy direct currents areprovided using appropriate heavy-current transformers and rectifiers.Due to heavy currents occurring, diode rectifiers are of disadvantage,because of the relatively high losses, and therefore active rectifierscomprising control elements, which are formed by respective transistors,are mainly employed. However, resistance welding devices with activerectifiers, for example synchronous rectifiers, also have relativelyhigh losses and, thus, relatively low efficiencies. Since significantline lengths and, thus, power losses incur in prior art by the usualseparate design of for example heavy-current transformer andrectification, a very poor efficiency is caused due to the heavycurrents.

For example, DE 10 2007 042 771 B3 describes a method for operating thepower supply of a resistance welding device by using a synchronousrectifier, through which power dissipation can be reduced and efficiencycan be improved.

In production lines in the automotive industry a plurality of spotwelding devices (often some 100 to 1000 individual units) is used toprepare various connections on body and chassis of the vehicle to bemanufactured. As the individual spot welding devices cause high lossesdue to the heavy-current transformers and power lines and circuitelements already, the total losses occurring in such production linesrange in extremely high dimensions, for example between 1 MW and 50 MW.Since the losses are mainly reflected in the form of heat loss, againmeasures must be taken to dissipate the heat, deteriorating the overallenergy balance even further.

Another disadvantage arises from the fact that very high connectedwattages of the electric grid are required because of the high losses ofsuch facilities, resulting in very high expenses for manufacturing,commissioning and operation of such a facility.

To produce a single spot weld with a welding current of 20 kA, aconnected wattage of the electric grid of up to 150 kW is for examplerequired according to prior art from a present point of view, whereinusing said welding current losses of up to 135 kW are generated,achieving a very poor efficiency of only some 10%.

SUMMARY OF THE INVENTION

Thus, the object of the present invention is to create a resistancewelding device by which the losses can be reduced and the energy balanceand the efficiency can be improved. Disadvantages of known devicesshould be reduced or avoided.

The object according to the invention is solved by an above-mentionedresistance welding device in which the power source, for providing thewelding current for welding the workpieces, is arranged at the weldinggun, and the power source comprises at least four contacts to form amulti-point contacting, in particular a four-point contacting, whereintwo first contacts of one polarity are connected to the one gun arm andtwo additional contacts of opposite polarity are connected to the othergun arm. Due to that multi-point contacting, lines which are usuallyrequired to connect the secondary side of the heavy-current transformeror the output of the synchronous rectifier to the gun arms or theelectrodes of the resistance welding device can be prevented or thelength thereof can be reduced and, thus, ohmic losses as well as contactlosses can considerably be reduced. Thus, preferably short lines withpreferably large cross-sections can be employed, while maintaining theflexibility of the welding gun. A further advantage lies in that due tosuch a contacting the losses, especially contact resistance losses, arereduced. Due to the at least four contacts (two contacts per polarity,this means two contacts for the plus pole and two contacts for the minuspole) the direct current to be transmitted can be halved, thereby alsocausing a reduction in contact losses. This also achieves that theactive contact areas can be substantially increased and thus in turn thecontact resistances are reduced. By means of the resistance weldingdevice according to the invention it is achieved that the connectedwattage for generating a welding current of 20 kA, for instance, can bereduced to only 75 kW (as compared to 150 kW with comparable prior artdevices), wherein losses of 60 kW occur. Hence, an efficiency that isapproximately twice as high, up to about 20% and more, can be achievedas compared to prior art.

Particular advantages arise when at least two contacts are connectedwithout lines and, thus, without contact resistances to the gun arm.This can be achieved in that those two contacts are virtually integratedin the power source and are connected to the corresponding parts of theresistance welding device, in particular the gun arms, directly, i.e.without laying of lines. Thus, by directly connecting a gun arm to thecontacts of the heavy-current transformer a connection without lines isachieved, whereas the second gun arm must be connected to the additionalcontacts by very short lines. This way a very high reduction of linelosses can be achieved, because the line length can be reduced tominimum. In prior art the heavy-current transformer is ideallypositioned as close to the welding gun as possible so that lines mustthen be laid from the heavy-current transformer to the welding gun,whereas in the solution according to the invention the heavy-currenttransformer is integrated in the welding gun while one gun arm ismounted on the heavy-current transformer directly so that only thesecond gun arm must be connected by one line or by two short lines. Ofcourse, for example sliding contacts or other connecting elements canalso be used instead of lines.

According to a further feature of the invention it is provided that thetwo first contacts of the one polarity and the additional two contactsof the opposite polarity are each arranged opposite each other, whereinthe two additional contacts are substantially arranged offset to eachother by 90° compared to the two first contacts. Such a geometricarrangement of the contacts at the power source makes it possible tocompletely avoid lines or to reduce their lengths and to thus reduceohmic losses as well as contact losses distinctly. Thus, contact facesthat are as large as possible can be provided. For example, in thedimensioning of a heavy-current transformer or a power source in orderto provide a direct current of 20 kA each of the four contacts has anarea from 15 cm×15 cm to 25 cm×25 cm, preferably 20 cm×20 cm.

Further reductions of losses can be achieved by integrating thesynchronous rectifier and the actuation circuit of the power source intothe heavy-current transformer and by parallel connecting several circuitelements of the synchronous rectifier and by connecting the circuitelements of the synchronous rectifier to the at least one secondarywinding of the heavy-current transformer without lines. Due to thisconstruction, no lines are required between the synchronous rectifierand the secondary side of the heavy-current transformer, wherebypossible ohmic losses and other losses occurring by using such lines arealso omitted. Possible supply circuits for the synchronous rectifier andthe actuation circuit are preferably also integrated into theheavy-current transformer. Preferably, the power unit in order to supplythe heavy-current transformer is positioned as close thereto as possibleto achieve connection lines and, thus, line losses and line inductancesas short as possible.

Advantageously, the circuit elements of the synchronous rectifier areformed of field-effect transistors, the drains of which are formed bythe housings thereof. The housings are connected to the at least onesecondary winding of the heavy-current transformer without lines. Thisarrangement thus provides that the secondary windings of theheavy-current transformer are contacted directly with the housings ofthe field-effect transistors, so that between these units no lines arerequired.

For achieving the required transmission ratio of the heavy-currenttransformer for generating the heavy secondary-side current, theheavy-current transformer comprises, in accordance with a furtherfeature of the invention, a plurality of, preferably at least 10,primary windings connected in series, and a plurality of, preferably atleast 10, secondary windings with center tapping connected in parallel.The primary current flows through the primary windings of theheavy-current transformer connected in series, while the relativelyheavy secondary-side current is divided between the plurality of,preferably at least 10, secondary windings connected in parallel. Thepartial currents on the secondary side are supplied to the correspondingcircuit elements of the synchronous rectifier. By using such a divisiona correspondingly high transmission ratio results despite low primaryand secondary numbers of turns. By using said construction, on theprimary side lower numbers of turns are required in contrast totraditional heavy-current transformers, whereby the length of theprimary winding can be decreased and thereby ohmic losses can bereduced. Because of the reduced number of turns of the primary windingand, thus, a resulting reduction of the line length, the leakageinductance of the heavy-current transformer, which is typical for thesystem, is in turn reduced, whereby the heavy-current transformer can beoperated at higher switching frequencies, for example 10 kHz. In turn,the higher switching frequencies cause a reduction of the overall sizeand the weight of the heavy-current transformer and, thus, advantageousinstallation options. So the heavy-current transformer can be positionedas close as possible to the electrodes of the resistance welding device.Thus, even the load of the robot can be reduced due to the low weight ofthe heavy-current transformer so that a small, more inexpensive robotwill suffice.

The transmission ratio of the heavy-current transformer is 10 to 1000,preferably at least 100, to ensure the generation of the heavysecondary-side current.

A particularly advantageous construction of the resistance weldingdevice can be achieved in that the heavy-current transformer comprisesan I-shaped of electrically conductive material, in the recesses ofwhich at least one respective ring core is arranged, wherein onerespective connection of each secondary winding is directly contactedwith an inner surface of the I-beam, and that the outer surfaces of theI-beam form the two first contacts of the power source. The I-beam thusforms the basis of the heavy-current transformer around which thesecondary windings are arranged such that connection lines are notrequired. The outer surfaces of the I-beam represent the two firstcontacts of the power source, which are connected to a gun arm of theresistance welding device directly, i.e. without lines. A space-savingarrangement is achieved in that the ring cores are not designedcircular, but oval. Preferably, closed ring cores are employed. By usingsaid design the series/parallel connection of the primary windings andthe secondary windings can be achieved by which the requiredtransmission ratio of the heavy-current transformer for the heavy directcurrent to be provided with reduced numbers of turns of the primarywindings and the secondary windings is achieved. Such a design isespecially useful when at least three secondary windings connected inparallel are arranged on each side of the I-beam.

Advantageously, the center tapping of the at least one secondary windingof the heavy-current transformer is connected to the I-beam withoutlines. Thus, corresponding lines between the individual components canbe omitted. By directly connecting the secondary winding to the centerof the I-beam a substantial increase of the connection surface is alsoachieved, and, thus, contact losses and line losses can again bereduced.

In the above-mentioned construction of the heavy-current transformerwith an I-beam, the at least one primary winding of the heavy-currenttransformer is arranged to extend through the at least one ring core, inparticular the ring cores arranged symmetrically on both sides in therecesses of the I-beam. By using such an arrangement of the primarywinding an optimum magnetic coupling to the secondary windings can beachieved.

As already mentioned above, the secondary windings are connectedelectrically with each other via the I-beam of the heavy-currenttransformer. The respective other connections of each secondary windingare connected via the synchronous rectifier and the actuation circuitpreferably directly with one contact plate each of electricallyconductive material, said contact plates being located above therecesses of the I-beam and the secondary windings arranged therein,wherein the outer surfaces of these contact plates form the twoadditional contacts of the power source.

The connections of the at least one primary winding of the heavy-currenttransformer are, in accordance with a further feature of the invention,led through at least one opening on an outer surface of the I-beam. Fromthere, the connections of the primary winding of the heavy-currenttransformer may be connected to the corresponding power source or thepower unit.

An advantageous embodiment of the resistance welding device results alsofrom that one secondary winding with center tapping of the heavy-currenttransformer is each formed by two metal sheets insulated from each otherand being of electrically conductive material and having a substantiallyS-shaped mirror-inverted course around the cross-section of a ring coreand through the ring core, wherein the exterior surfaces of the metalsheets form the contacts for the connection with the circuit elements ofthe synchronous rectifier or the electrodes. This achieves an extremelyspace-saving and compact construction. At the same time, very largecontact faces are available for the connection of the secondary windingwith the center or the central web of the I-beam and the circuitelements of the synchronous rectifier so as to ensure the heavy currentflow with as little losses as possible.

The metal sheets for forming the secondary winding of the heavy-currenttransformer are insulated from each other by an insulating layer, forinstance, a paper layer. Thus, it is possible to arrange two secondarywindings on one ring core and to thus reduce the overall size, theweight and the losses considerably. By such an arrangement a centerrectifier is implemented on the secondary side, wherein the I-beam isforming the center with the one connected, in particular soldered, endof the secondary windings.

The I-beam and the contact plates of the heavy-current transformerpreferably form a cube or ashlar-shaped unit, wherein an electricalinsulation is arranged between the I-beam and the contact plates. Thetwo outer surfaces of the I-beam represent the two first contacts andthe two outer surfaces of the contact plates represent the twoadditional contacts of the power source which are offset by 90° thereto.If all further components of the power source for providing the weldingcurrent, such as the synchronous rectifier, the actuation circuit, thesupply circuits for the synchronous rectifier and the actuation circuitare integrated into this cube or ashlar-shaped unit, an autonomous unitis formed whose input side merely has to be connected with the powerunit and whose output side merely has to be connected with the gun armsor electrodes of the resistance welding device. Usual lines between theindividual circuits of the power source may be omitted or at least theirlength may be substantially reduced.

If cover plates are arranged on the front faces of the I-beam, a sturdyunit of the heavy-current transformer in cube or ashlar-shape may beformed.

If the cover plates are also formed of an electrically conductivematerial and are adapted to be screwed to the contact plates, anelectrical connection of the contact plates may be achieved. Thus, aseparate electrical line connecting the two contact plates electricallywith one another may be omitted to establish a voltage or potentialequalization and to thus avoid unbalance of the two contact plates. Thecover plates thus establish the electrical connection of the two contactplates of the symmetric arrangement of the heavy-current transformer orthe power source for providing the welding current for the resistancewelding device.

The I-beam and/or the contact plates and/or the cover plates and/or themetal sheets for forming the secondary winding of the heavy-currenttransformer are preferably formed of copper or a copper alloy,preferably with a silver coating. Copper or copper alloys have optimumelectrical properties and exhibit good thermal conductivity, wherebythermal losses occurring can be discharged faster. The silver coatingprevents the copper or the copper alloy from oxidizing. Instead ofcopper or copper alloys also aluminum or aluminum alloys come intoconsideration, which have a weight advantage over copper, although theresistance to corrosion is not that high. Instead of a silver coatingalso a coating of tin and other materials or compounds or layers thereofis possible.

The I-beam and/or the contact plates may also be formed at leastpartially by the gun arms of the resistance welding device. This enablesa construction to be formed in which the power source for providing thewelding current of the resistance welding device is at least partiallyintegrated therein.

On the outer surfaces of the I-beam as well as on the outer surfaces ofthe contact plates connection devices, preferably bore holes with athread to receive screws, can be arranged. Via these connection devicesboth the mechanical and the electrical connection of the power source tothe components of the resistance welding device, in particular the gunarms, or other devices to be supplied with the direct current can beestablished. Moreover, various other elements may be attached to theouter surfaces of the I-beam or the contact plates by such connectiondevices.

In accordance with a further feature of the invention, at theconnections of a secondary winding a current transducer each is arrangedfor measuring the current through this secondary winding, said currenttransducer being connected with the actuation circuit. The currenttransducer measures the secondary-side current due to which the circuitelements of the synchronous rectifier are activated to minimize theconduction losses and the switching losses. In the case of a symmetricalarrangement of the power source with an I-beam as a basis of theheavy-current transformer, secondary windings are arranged on both sidesof the I-beam, and the current transducers are also arranged on bothsides. The current transducers are each contacted directly with theactuation circuit that is arranged next to it and is connected with therespectively opposite actuation circuit via appropriate lines. It isessential that, due to the parallel connection of the secondarywindings, always the same current is flowing in each winding and thecurrent flow thus has to be tapped from one secondary winding only inorder to make a conclusion with respect to the entire current flow. In aparallel connection of ten secondary windings only one tenth of theentire secondary-side current flow is therefore measured by the currenttransducers, which is why these can be sized substantially smaller.Thus, in turn, a reduction of the overall size of the heavy-currenttransformer or the power source is achieved.

It is advantageous to have the current transducers arrangedsubstantially at an orientation of 90° to the direction of the weldingcurrent since thereby interferences by the magnetic field induced by thewelding current and, thus, measurement errors can be reduced. Thus, avery precise measurement can be performed.

In order to avoid interferences from external magnetic fields, eachcurrent transducer is shielded by a housing and preferably a shieldingof a magnetically conductive material. For such shieldings ferrites areespecially suited materials.

In order to minimize the conduction losses and switching losses of thecircuit elements of the synchronous rectifier, the actuation circuit isdesigned to activate the circuit elements of the synchronous rectifierat a preset point in time before reaching zero crossing of the currentin the secondary winding. By means of this preset point in time it ispossible to equalize the delay occurring from the detection of the zerocrossing of the secondary-side current to the activation of therespective circuit elements. This means that the switch on and switchoff times of the circuit elements of the synchronous rectifier are notdetermined by the zero crossing of the secondary-side current, but bythe achievement of defined switch on and switch off thresholds. Theswitch on and switch off thresholds are defined according to theswitching delays to be expected. The switch on and switch off thresholdsare at best designed adjustable, in order to further reduce the losses.In a heavy-current transformer of 20 kA the switching time can forexample be set 100 ns prior to zero crossing so that all components, inparticular the circuit elements of the synchronous rectifier, need to beswitched within this time period.

For discharging the thermal losses occurring in the power source of theresistance welding device, channels for guiding a cooling fluid arepreferably arranged in the I-beam and the contact plates. As a coolingfluid, water is in particular suitable, but gaseous cooling agents mayalso be transported through the cooling channels and the thermal lossesmay be discharged therethrough.

A preferred embodiment of the cooling channels is given in that twoinlets for feeding the cooling fluid and one outlet for discharging thecooling fluid are arranged at an outer surface of the I-beam, whereinthe cooling channels are arranged to extend from each inlet to thecontact plates and via the I-beam to the outlet. The cross-section ofthe outlet corresponds to the sum of the cross-sections of all theinlets. This course of the cooling channels achieves that first of allthe contact plates at which the circuit boards of the synchronousrectifier and of the actuation circuit with the more sensitivecomponents are arranged, are cooled with the correspondingly coldcooling fluid. Then, the less sensitive components, in particular theparts of the heavy-current transformer, namely the I-beam being inconnection with the secondary windings, are cooled.

The actuation circuit and the synchronous rectifier are preferablyarranged on at least one circuit board which is arranged on the innersurface of at least one contact plate. This arrangement of the actuationcircuit and of the synchronous rectifier on the inner surface of atleast one contact plate enables a direct contacting, i.e. a contactingwithout lines, of the connections of the secondary windings with thecircuit elements of the synchronous rectifier and also a directcontacting or a contacting without lines of the outputs of thesynchronous rectifier with the contact plate. Preferably, theheavy-current transformer or the power source for providing the weldingcurrent is constructed symmetrically, wherein one circuit board eachwith a part of the synchronous rectifier and the actuation circuit isarranged at both sides of the symmetrically arranged secondary windingsbelow a respective contact plate.

Each circuit board of the synchronous rectifier and of the actuationcircuit comprises preferably openings above which the circuit elementsare arranged, and the inner surfaces of the contact plates, in theplaces of the openings in the circuit board of the synchronousrectifier, comprise protrusions, in particular pinnacle-shapedprotrusions, so that the circuit elements can be contacted without linesvia the protrusions protruding into the openings on the circuit board onthe inner surface of the contact plates. Thus, connection lines betweenthe circuit elements of the synchronous rectifier and the contact platescan be omitted, whereby on the one hand the ohmic losses can be reducedand on the other hand the thermal transition between the circuitelements and the contact plates can be improved. Finally, the productioneffort is also reduced since no connection lines have to be laid andconnected, but the circuit elements are connected, preferably soldered,with the protrusions directly. A simple positioning of the circuit boardcan also be enabled by the protrusions and thus manufacturing can befacilitated substantially.

A design in which the source connections of the circuit elements formedby field-effect transistors are, via the protrusions, in particularpinnacle-shaped protrusions, directly connected electrically andthermally with the contact plate is also of advantage since here againthe corresponding lines can be avoided.

Each circuit board is preferably arranged between the I-beam and thecontact plates for forming the necessary electrical insulation. Thus, itis not necessary to provide a separate electrical insulation between theI-beam and the contact plates.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail with the help of theaccompanying drawings.

In which:

FIG. 1 is a prior art resistance welding device with a robot and awelding gun secured thereto in schematic illustration;

FIG. 2 is a schematic block diagram of a resistance welding device witha power source in order to provide the welding current;

FIG. 3 is a resistance welding device, in particular a welding gun withan integrated power source in order to provide the welding current inschematic illustration;

FIG. 4 is a schematic block diagram of the power source in order toprovide the welding current;

FIG. 5 shows an embodiment of the power source in order to provide adirect current;

FIG. 6 shows the power source according to FIG. 5 in an exploded view;

FIG. 7 shows the power source according to FIG. 5 with plotted course ofthe cooling channels;

FIG. 8 is a view of the I-beam of the heavy-current transformer of thepower source;

FIG. 9 shows the I-beam according to FIG. 8 in section;

FIG. 10 is a contact plate of the heavy-current transformer of the powersource including the printed circuit board of the synchronous rectifierand the actuation circuit;

FIG. 11 shows the contact plate according to FIG. 10 in section;

FIG. 12 shows a secondary winding of the heavy-current transformer withcurrent transducer in exploded view;

FIG. 13 shows the design of a secondary winding of the heavy-currenttransformer in an exploded view;

FIG. 14 is a block diagram of a circuit in order to supply thesynchronous rectifier and the actuation circuit with electric energy;

FIG. 15 shows a time course of the supply voltage of the supply circuitaccording to FIG. 14; and

FIG. 16 shows time courses in order to illustrate the actuation of thecircuit elements of a synchronous rectifier depending on thesecondary-side currents of the heavy-current transformer.

DETAILED DESCRIPTION OF THE DRAWINGS

In the embodiment illustrated in FIGS. 1 to 16 a design of a resistancewelding device 1 with essential components is described. In the figuressame parts are denoted with same reference characters.

In FIG. 1 a resistance welding device 1 for resistance welding of atleast two workpieces 2, 3 having a robot for manipulation is shown inperspective. The resistance welding device 1 is composed of a weldinggun 4 attached to the robot and having two gun arms 5, to which holders6 for holding an electrode 7 each are arranged. The electrodes 7 areeach circulated by a band 8 which reduces the contact resistance inresistance welding and protects the electrodes 7. Moreover, theresulting image on band 8 of the spot weld produced can be analyzed andused to evaluate the weld quality. The band 8 for protection of theelectrodes 7 is unwound from a winding device 9 which can be arranged onthe welding gun 4 or gun arms 5, respectively, and is guided back alongthe gun arms 5, the electrode holders 6 and the electrodes 7 to thewinding device 9, where the band 8 is re-coiled. To perform the spotwelding, the welding current, which is supplied by a corresponding powerunit 19, is conducted through the electrodes 7. Thereby the workpieces2, 3 are connected together by a spot weld produced during the spotwelding process. Usually, the power unit 19 in order to provide thewelding current is located outside the resistance welding device 1, asschematically illustrated in FIG. 1. The welding current is supplied tothe electrodes 7 or gun arms 5, which are formed electricallyconductive, via appropriate lines 11. Since the amplitude of the weldingcurrent is in the range of some kA, correspondingly large cross-sectionsfor the lines 11 are required, resulting in correspondingly high ohmiclosses.

Furthermore, long primary supply lines lead to an increased inductanceof the lines 11, and therefore the switching frequency at which aheavy-current transformer 12 of a power source 10 is operated islimited, resulting in very large heavy-current transformers 12. In priorart the power unit 19 is positioned in a switching cabinet next to thewelding robot, so that very long supply lines, for example of up to 30m, to the heavy-current transformer 12 are required for the welding gun4 on the robot.

In the solution according to the invention a significant reduction inweight and size is achieved, so that a positioning of the power unit 19directly on the robot, in particular in the section of the gun holder,is enabled. In addition, the power unit 19 is preferably designed to bewater cooled.

FIG. 2 shows a schematic block diagram of a resistance welding device 1with a power source 10 in order to provide the welding current. Althoughin the embodiment shown the power source 10 is used to provide thewelding current for the resistance welding device 1, the power source10, in particular the entire design of the power supply, can also beused to provide a direct current for other applications. The powersource 10 includes a heavy-current transformer 12 having at least oneprimary winding 13, at least one secondary winding 14 with centertapping and a ring core 15. The current transformed by means of theheavy-current transformer 12 is rectified in a synchronous rectifier 16and supplied to the gun arms 5 or the electrodes 7 of the resistancewelding device 1. To control the synchronous rectifier 16, an actuationcircuit 17 is provided. The actuation circuit 17 sends correspondingtrigger pulses to the circuit elements 24 of the synchronous rectifier16 based on the secondary-side currents of the heavy-current transformer12 measured, for example, via the current transducers 18.

As is generally known, due to the heavy welding currents as a result ofthe sum of the required line length, both significant ohmic and/orinductive losses as well as conduction and switching losses occur in thecircuit elements 24 of the synchronous rectifier 16. Besides, also inthe rectifier, in the power supply for the synchronous rectifier 16 andthe actuation circuit 17 losses occur. Accordingly, the resultingefficiency of such resistance welding devices 1 is low.

To produce the primary-side current of the heavy-current transformer 12,a power unit 19 is provided, which is arranged between an electric gridand the power source 10. The power unit 19 provides the primary-sidecurrent to the heavy-current transformer 12 or the power source 10 withthe desired amplitude and the desired frequency.

FIG. 3 shows a resistance welding device 1 with integrated power source10 in a schematic illustration. The power source 10 is arrangeddirectly, in particular as a supporting member, on the welding gun 4 orthe gun arms 5 of the resistance welding device 1, so that at least onepart of the lines 11 in order to guide the welding current to theelectrodes 7 can be omitted and, thus, the line lengths aresignificantly shortened, since connection with one gun arm 5 is requiredonly. The power source 10 has at least four contacts 20, 21, 22, 23 toform a multi-point contacting, wherein two first contacts 20, 21 of onepolarity are connected to the one gun arm 5 and two additional contacts22, 23 of opposite polarity are connected to the other gun arm 5.Advantageously, the two first contacts 20, 21 of the one polarity andthe other two contacts 22, 23 of the other polarity are each arrangedopposite each other, wherein the two other contacts 22, 23 aresubstantially arranged offset to each other by 90° compared to the twofirst contacts 20, 21. By that multi-point contacting lines, which areusually required to connect the secondary side 14 of the heavy-currenttransformer 12 to the gun arms 5 or the electrodes 6 of the resistancewelding device 1, can be prevented or the length thereof can be reducedand, thus, ohmic losses as well as contact losses can considerably bereduced. Thus, preferably short lines with preferably largecross-sections can be employed, while maintaining the flexibility of thewelding gun 4. A further advantage lies in that due to such a contactingthe losses, especially contact resistances, are reduced. Due to the atleast four contacts 20, 21, 22, 23 the welding current to be transmittedcan be halved, thereby also causing a reduction in contact losses, sincedue to the substantial increase in the active contact areas contactresistances are reduced. For example, in the dimensioning of aheavy-current transformer 12 or power source 10 in order to provide adirect current of 20 kA each of the four contacts 20, 21, 22, 23 has anarea from 15 cm×15 cm to 25 cm×25 cm, preferably 20 cm×20 cm.

In the embodiment shown the power source 10 is formed substantiallycube-shaped, wherein the side surface of the cube forms the contacts 20,21, 22, 23. The two first contacts 20, 21 are connected to one electrode7 and the two other contacts 22, 23 are connected to the other electrode7 of the resistance welding device 1 via the gun arms 5. As can be seenin the partially exploded view, at least one gun arm 5, in particularthe lower gun arm 5, is connected via a supporting member 23 a of thelower gun arm 5, while the other, in particular the upper gun arm 5, isconnected to the additional contacts 22, 23 via a flexible connectorclamp 23 b. At least one gun arm 5 is, thus, connected to theheavy-current transformer 12 directly and the other gun arm 5 isconnected thereto via a very short line, for example less than 50 cm.With the lines 11 between the power source 10 and the electrodes 7 orthe gun arms 5 of the resistance welding device 1 being omitted or beingparticularly short, ohmic and inductive losses can be considerablyreduced.

Particular advantages arise when at least two contacts 20, 21 areconnected directly or without lines and, thus, without contactresistances to a gun arm 5. This can be achieved in that those twocontacts 20, 21 are virtually integrated in the power source 10 and areconnected to corresponding parts of the resistance welding device 1, inparticular the gun arms 5 directly, i.e. without laying of supply lines.Thus, by directly connecting a gun arm 5 to the contacts 20, 21 of theheavy-current transformer 12 a connection without lines is achieved,whereas the second gun arm 5 must be connected to the contacts 22, 23 byvery short lines. This way a very high reduction of line losses can beachieved, because the line length is reduced to minimum. In prior artthe heavy-current transformer is ideally positioned as close to thewelding gun 4 as possible so that lines must then be laid from theheavy-current transformer 12 to the welding gun 4, whereas in thesolution according to the invention the heavy-current transformer 12 isintegrated in the welding gun 4 while one gun arm 5 is mounted on theheavy-current transformer 12 directly so that only the second gun arm 5must be connected by one or two short lines. Of course, for examplesliding contacts or other connecting elements can also be used insteadof lines. Also losses within the power source 10 can be reducedsignificantly due to the compact design and the direct connection, i.e.,connection without lines, of the components of the power source 10.

Advantageously, all components of the power source 10, including thesynchronous rectifier 16, the actuation circuit 17, the currenttransducer 18 and all supply circuits for the synchronous rectifier 16and the actuation circuit 17 are included in the cube or ashlar-shapedunit. That is, by the integration of the electronic components/circuitsa structural unit is created in the form of a cube, where the user onlyneeds to provide energy in the form of corresponding alternating voltageor corresponding alternating current on the primary side, in order toobtain a direct current or direct voltage on the secondary sideappropriately dimensioned with high performance. The control andregulation is autonomously carried out in the cube or the power source10. So the cube and the power source 10 are versatilely applicable forsupplying components with heavy direct current. In particular, the powersource 10 serves to supply a low voltage and heavy current, as usual inresistance welding processes.

When used in a resistance welding process, parts of the cube-shapedpower source 10 can also be formed by components of the resistancewelding device 1, for example parts of the gun arms 5 or the like, asshown. The cube or power source 10 takes on a supporting function byattaching a gun arm 5 directly to the cube. The other gun arm 5 iscontacted via connection lines (not shown). By using said design, longsupply lines can be prevented, so that a substantial reduction of thelosses is obtained. However, to have the cube integrated into such awelding gun 4, it is required to keep the size thereof as small aspossible. For example, in a dimensioning of the direct current to beprovided of up to 20 kA the cube or power source 10 has a side length ofbetween 10 cm and 20 cm, particularly 15 cm. By using said compactdesign of the cube-shaped power source 10, it is easily possible tointegrate this, for example, in the base body of the welding gun 4.

FIG. 4 shows a schematic block diagram of the power source 10 in orderto provide a direct current, in particular a welding current. In thispreferred embodiment of the power source 10 ten primary windings 13 ofthe heavy-current transformer 12 are connected in series and tensecondary windings 14 of the heavy-current transformer 12 with centertapping are connected in parallel. By using such a design of theheavy-current transformer 12, the corresponding high transmission ratioin order to achieve a corresponding heavy current on the secondary sidecan be achieved even with low numbers of turns of the primary windings13 and low numbers of turns of the secondary windings 14. For example, atransmission ratio of 100 can be achieved with ten primary windings 13and also ten secondary windings 14. The primary current flows throughthe primary windings 13 of the heavy-current transformer 12 connected inseries, while the relatively heavy secondary-side current is dividedbetween the ten secondary windings 14 connected in parallel. The partialcurrents on the secondary side are supplied to corresponding circuitelements 24 of the synchronous rectifier 16. By using such a division acorrespondingly high transmission ratio (here 100) results despite lowprimary and secondary numbers of turns. By using said construction, onthe primary side lower numbers of turns are required in contrast totraditional heavy-current transformers, whereby the length of theprimary winding 13 can be decreased and thereby ohmic losses can bereduced. Because of the reduced number of turns of the primary winding13 and, thus, a resulting reduction of the line length, the leakageinductance of the heavy-current transformer 12, which is typical for thesystem, is in turn reduced, whereby the heavy-current transformer 12 canbe operated at higher switching frequencies, for example 10 kHz. Inturn, the higher switching frequencies compared to conventionalheavy-current transformers cause a reduction of the overall size and theweight of the heavy-current transformer 12 and, thus, advantageousinstallation options. So the heavy-current transformer 12 can, forexample, be positioned very close to the electrodes 7 of a resistancewelding device 1. Thus, even the load of the welding robot can bereduced due to the lower weight of the heavy-current transformer 12 sothat a small, more inexpensive welding robot will suffice.

Conventional transformers, in which there is no series/parallelconnection of the primary and secondary windings, would requirecorrespondingly more primary windings, which would result insubstantially longer wire lengths on the primary side. Due to largerwire lengths ohmic losses increase on the one hand and a higher leakageinductance results on the other hand, which is why the frequencies, withwhich the prior art transformer can be operated, are limited to somekHz.

In contrast, in the construction of the heavy-current transformer 12described herein the ohmic losses and the leakage inductance of theprimary windings 13 and secondary windings 14 inherent in the system arelow, which is why frequencies in the range of 10 kHz and above can beused. Thereby, a substantially smaller overall size of the heavy-currenttransformer 12 can in turn be achieved. The smaller overall size of theheavy-current transformer 12 or the power source 10 makes it in turnpossible to arrange it more closely to the location, where the currentproduced is required, for example on the gun arms 5 of a resistancewelding device 1.

By parallel-connecting the secondary windings 14 of the heavy-currenttransformer 12 the resulting heavy current on the secondary side isdivided into several partial currents. These partial currents aretransmitted to the circuit elements 24 of the synchronous rectifier 16,as schematically illustrated. To activate the circuit elements 24 anactuation circuit 17 is provided, drawn in in the section of the primarywinding 13 and the secondary winding 14, wherein both the synchronousrectifier 16 and the actuation circuit 17 including associated sensorsare arranged within the cube, that is within the heavy-currenttransformer 12. The synchronous rectifier 16 and the actuation circuit17 are formed and dimensioned such that they perform regulation andcontrol of the power source 10 autonomously, that is without externalinfluence. Therefore, the cube has preferably no control lines forintervention from outside, but merely connections or contacts for theprimary-side supply and connections or contacts for the delivery of thesecondary-side electric energy generated, in particular the heavysecondary direct current.

However, it is possible that a corresponding connection of the actuationcircuit 17 is led through to provide the actuation circuit 17 with givenvalues. By making external adjustments the power source 10 can ideallybe adjusted to the field of application. As known from prior art,systems for changing or transmitting data can, however, be employedwhich operate wirelessly, preferably inductively, magnetically or viaBluetooth, so that no control connection needs to be led through.

Control and/or regulation of the power source 10 is done via theintegrated sensors. By measuring the secondary-side currents of asecondary winding 14 using corresponding current transducers 18 theactuation circuit 17 obtains the information at which points in time thecircuit elements 24 of the synchronous rectifier 16 should be switched.Because the current transducers 18 only measure a fraction, here onetenth, of the secondary-side current of the heavy-current transformer12, they can be designed smaller, again positively affecting the overallsize of the power source 10.

To reduce the conduction and switching losses the circuit elements 24 ofthe synchronous rectifier 16 are preferably switched in zero crossing ofthe secondary-side currents through the secondary windings 14 of theheavy-current transformer 12. Since certain delays occur from thedetection of the zero crossing of the secondary-side current by thecurrent transducers 18 to the activation of the circuit elements 24 ofthe synchronous rectifier 16, according to the invention the actuationcircuit 17 is formed to switch the circuit elements 24 of thesynchronous rectifier 16 at a preset point in time prior to reachingzero crossing of the current in the secondary winding 14. Thus, theactuation circuit 17 causes the switching of the circuit elements 24 ofthe synchronous rectifier 16 at a point in time at which the currents inthe secondary winding 14 of the heavy-current transformer 12 measured bythe current transducers 18 fall below or exceed certain switch-on andswitch-off thresholds. By using said method it can be achieved that thecircuit elements 24 of the synchronous rectifier 16 are substantiallyswitched during zero crossing of the currents through the secondarywinding 14 of the heavy-current transformer 12, whereby conduction andswitching losses can be minimized (also see FIG. 16).

In FIG. 4 also the supply circuit 48 for supplying the synchronousrectifier 16 and the actuation circuit 17 with electric energy is drawnin for a primary winding 13 and a secondary winding 14. Also this supplycircuit 48 is preferably integrated in the power source 10, that is inthe cube. Since the supply of the synchronous rectifier 16 and theactuation circuit 17 of the power source 10 with sufficient electricenergy must be ensured at the time when the delivery of the directcurrent, for example the welding current, is desired, a sufficientlyfast activation of the supply circuit 48 is required (see FIG. 15) or itis configured such that with the activation of the power source 10 asufficiently high supply voltage is provided as fast as possible andsubsequently the required power or the required current is delivered.

FIG. 5 shows the embodiment of the power source 10 according to FIG. 3in enlarged illustration. The power source 10 in order to provide adirect current, for example a welding current, has substantially theform of a cube or ashlar, wherein the lateral faces of the cube orashlar represent the contacts 20, 21, 22, 23 via which the directcurrent generated can be transmitted to the relevant consumer load, suchas the gun arms 5 or the electrodes 7 of a resistance welding device 1.All components of the power source 10, that is the heavy-currenttransformer 12, the synchronous rectifier 16, the actuation circuit 17,the current transducers 18, the supply circuit 48, etc. are included orintegrated in this cube or ashlar-shaped member of the power source 10.By using said compact design the losses of the power source 10 can bekept particularly low and, thus, the efficiency thereof cansubstantially be increased, since an optimum shortening of lines and,thus, switching times is achieved in the cube by the integration of theelectronic components, in particular of the printed circuit boardscomprising the synchronous rectifier 16, the actuation circuit 17 andthe supply circuit 48. By integrating the synchronous rectifier 16 andthe actuation circuit 17 as well as the supply lines 48 of the powersource 10 into the heavy-current transformer 12 and by parallelconnecting several circuit elements 24 of the synchronous rectifier 16and by connecting the circuit elements 24 to the secondary windings 14of the heavy-current transformer 12 without lines, no lines are requiredbetween the synchronous rectifier 16 and the secondary side 14 of theheavy-current transformer 12, whereby possible ohmic losses and otherlosses occurring by using such lines are also omitted. The power unit 19in order to supply the heavy-current transformer 12 is positioned asclose thereto as possible to achieve connection lines and, thus, linelosses and line inductances as short as possible. By integrating allcomponents an autonomous unit is formed, which on the input side has tobe connected to the power unit 19 and on the output side—in case of aresistance welding device 1—to the gun arms 5 or the electrodes 7 only.Common lines between the single circuits of the power source 10 are nolonger needed or at least are substantially reduced in the lengththereof.

The basis of the heavy-current transformer 12 of the power source 10 isa transformer element in the form of an I-beam 25 made of electricallyconductive material, in particular copper or copper alloy, at best witha coating, for example of silver. In the recesses 25 a of the I-beam 25ring cores 15 comprising the secondary windings 14 of the heavy-currenttransformer 12 are arranged on both sides. In terms of space it isadvantageous when the ring cores 15 do not have a circular, but oval orflat cross-section. In the embodiment shown, in every recess 25 a of theI-beam 25 five ring cores 15 are each arranged in parallel with therespective secondary windings 14. The primary winding 13 or the primarywindings 13 interconnected in series (dash-dotted line) extend throughthe ring cores 15 arranged in the recesses 25 a of the I-beam 25 andaround the web of the I-beam 25. By using that course of the primarywinding 13 and by the ring cores 15 arranged particularly symmetricallyin the two recesses 25 a of the I-beam 25 an optimum magnetic couplingto the secondary windings 14 can be achieved. Connections 26 of theprimary winding 13 are led through at least one opening 27 on an outersurface 28 of the I-beam 25. The primary winding 13 of the heavy-currenttransformer 12 can be connected to the corresponding power unit 19 viasaid connections 26. The outer surfaces 28 of the I-beam 25 form the twofirst contacts 20, 21 of the power source 10, which, for example, areconnected to the one of the electrodes 7 of the resistance weldingdevice 1.

Above the recesses 25 a of the I-beam 25 contact plates 29 are located,the outer surfaces thereof forming the other two contacts 22, 23 of thepower source 10 and being insulated against the I-beam 25. The contactplates 29 are also made of electrically conductive material, for examplecopper or a copper alloy, at best with a coating, for example made ofsilver. Copper or copper alloys have optimum electrical properties andexhibit a good thermal conductivity, whereby thermal losses occurringcan be discharged faster. The silver coating prevents the copper or thecopper alloy from oxidizing. Instead of copper or copper alloys alsoaluminum or aluminum alloys come into consideration, which have a weightadvantage over copper, although the resistance to corrosion is not thathigh. Instead of a silver coating also a coating of tin and othermaterials or compounds or layers thereof is possible. Between thecontact plates 29 and the corresponding connections of the secondarywindings 14 of the heavy-current transformer 12 the circuit boards 35 ofthe synchronous rectifier 16 and the actuation circuit 17 are arranged.Said circuit boards 35 or printed circuit boards are mounted or solderedto the contact plates 29 directly and will subsequently be attached tothe I-beam 25 in an insulated manner. By using said design thesecondary-side connections of the heavy-current transformer 12 can beconnected or contacted to the circuit elements 24 of the synchronousrectifier 16 directly without having to lay lines. The outputs of thesynchronous rectifier 16 are also preferably directly connected to thecontact plates 29, whereby lines are not required. The contact plates 29are connected to the I-beam 25, preferably screwed in place (not shown).On the outer surfaces 28 of the I-beam 25 as well as on the outersurfaces of the contact plates 29 connection devices 30, such as boreholes with corresponding threads to receive screws, can be arranged. Forexample the lines to the gun arms 5 of a resistance welding device 1 orother devices to be supplied with the direct current can be attached viasaid connection devices 30, or a gun arm 5 can be attached directly tothe I-beam 25 or to the contact plates 29.

Cover plates 31 can be attached to the upper side and the lower side ofthe cube or ashlar-shaped power source 10 and can be connected, forexample screwed (see FIG. 6), to the I-beam 25 and the contact plates29. Preferably, the cover plates 31 are also made of electricallyconductive material and screwed to the contact plates 29, resulting in asturdy unit of the heavy-current transformer 12 as well as producing anelectric connection between the contact plates 29 via the cover plates31. Thus it is achieved that a charge equalization can take place viathe cover plate 31, and, thus, unbalanced loads of the heavy-currenttransformer 12 will not occur. Thus, a separate electric line whichwould connect the two contact plates 29 electrically with each other canbe omitted in order to create the voltage and potential equalization andto avoid imbalances. That means that the electrical connection of bothcontact plates 29 of the balanced layout of the heavy-currenttransformer 12 or the power source 10 in order to provide the weldingcurrent is established via the cover plates 31. In this case, of course,an appropriate insulation over the I-beam 25 needs to be provided for.The contact plates 31 as well as the I-beam 25 and the contact plates 29are preferably made of copper or a copper alloy, preferably with asilver coating.

On an outer surface 28 of the I-beam 25, in particular the first contact20, two inlets 32 are disposed for feeding a cooling fluid and an outlet33 for discharging the cooling fluid, in order to allow for cooling thecomponents of power source 10. The cross-section of the outlet 33 fordischarging the cooling fluid exhibits the sum of the cross-sections ofall inlets 32 for feeding the cooling fluid. For an optimum course ofthe cooling fluid the cooling channels 39 are correspondingly arranged(see FIGS. 9 and 11). As a cooling fluid water or another liquid, butalso a gaseous cooling agent, can be used.

As can be seen in the exploded view of the power source 10 according toFIG. 6, the current transducers 18 for measuring the secondary-sidecurrents of the heavy-current transformer 12 are directly positioned onthe secondary windings 14 arranged on top, which means that on eachfirst or topmost secondary winding 14 on both sides of the I-beam 25 acurrent transducer 18 is arranged such that the current through thissecondary winding 14 can be determined due to the current induced. Toavoid manipulation of the currents measured by the current transducers18 by external magnetic fields, preferably a housing 34 made ofmagnetically conductive material, for example ferrites, is arrangedabove the current transducers 18 for shielding.

The current transducers 18 are arranged on both sides of the I-beam 25on each of the first and second secondary winding 14. Due to currentflowing through the primary windings 13 the current discharges on oneside of the I-beam 25, whereby the topmost secondary winding 14, thus,forms the first secondary winding 14, whereas on the opposite side thecurrent is now entering into the topmost secondary winding 14 and, thus,forms the second secondary winding. By using a full bridge it isrequired to detect the current flow from the first and second secondarywindings 14 always independently from each other, so that depending onthe current the corresponding circuit elements 24 of the synchronoustransformer 16 can be actuated. Thus, it is possible to actuate thecircuit elements 24 of both sides of the I-beam 25 nearly synchronouslyby a control pulse induced by the current transducer 18.

Between the contact plates 29 and the I-beam 25 the circuit boards 35 ofthe synchronous rectifier 16 and the actuation circuit 17 are arranged.Simultaneously the circuit boards 35 establish the required insulationbetween the I-beam and the contact plates 29. The corresponding circuitelements 24 of the synchronous rectifier 16 are directly contacted withthe secondary windings 14 of the heavy-current transformer 12. Viacorresponding protrusions 36, in particular pinnacle-shaped protrusions,on the inner side of the contact plate 29 and corresponding openings 37on the circuit board 35 below the circuit elements 24 a directcontacting of the circuit elements 24 with the contact plates 29 can bemade. The circuit elements 24 are preferably formed of suitablefield-effect transistors, the drains of which are formed by the housingsthereof. The housings of the field-effect transistors are connected toat least one secondary winding 14 of the heavy-current transformer 12directly or without lines, so that between these units no lines arerequired. For example, field-effect transistors made of silicon orgallium nitride are employed. The current transducers 18 are directlyconnected to the circuit board 35 of the synchronous rectifier 16 andthe actuation circuit 17 arranged alongside and are connected to theopposite circuit board 35 of the synchronous rectifier 16 and theactuation circuit 17 via an appropriate line 38.

The assembly of the power source 10 according to FIGS. 5 and 6 ispreferably performed by a soldering process using two differentsoldering temperatures. At first the secondary windings 14 are connectedto the recesses 25 a of the I-beam 25 using a soldering material, inparticular a soldering tin melting at a first, higher temperatureT_(s)1, for example 260° C. Also the contact plates 29 are contactedwith the circuit boards 35 using a soldering material melting at thefirst, higher melting temperature T_(s)1, for example 260° C. Then inturn using a soldering material melting at the first melting temperatureT_(s)1, for example 260° C., the components of the synchronous rectifier16 and the actuation circuit 17 are mounted to the circuit board 35.Because of the capillary action of the circuit board 35 on the contactplate 29 there is no risk of loosening of the circuit board 35 from thecontact plate 29. Following these steps the external contacts of thesecondary windings 14 and the contacts on the circuit boards 35 aresprinkled with soldering material at a second melting temperature T_(s)2which is lower compared to the first melting temperature T_(s)1, forexample 180° C., the contact plates 29 comprising the circuit boards 35are connected, preferably screwed, to the I-beam 25, and subsequentlyheated using the second melting temperature T_(s)2 of the solderingmaterial, for example 180° C., so that the connection of the secondarywindings 14 to the circuit elements 24 of the synchronous rectifier 16is established. By using a soldering material of said second, lowermelting temperature T_(s)2 it can be ensured that the soldered jointsproduced using the soldering material of higher melting temperatureT_(s)1 are not fused or become highly resistive by crystallizationprocesses. Finally, the primary winding 13 is threaded through the ringcores 15 and subsequently the power transformers 18 are mounted andcontacted and the line 38 is laid. By attaching the cover plates 31 thepower source 10 is accomplished. To reduce tensile and bending forces tothe components of the power source 10, all cavities can be encapsulatedprior to assembly of the cover plates 31. Via openings providedtherefore (not shown) for example in the cover plates 31 anencapsulation can also be performed following the assembly of the coverplates 31.

FIG. 7 shows the power source 10 of FIGS. 5 and 6 illustrating thecourse of the cooling channels 39 (dashed line). Accordingly, thecooling channels 39 at first extend from the two inlets 32 arrangedsymmetrically into the contact plates 29 where the most powerful heatsources (the circuit elements 24 of the synchronous rectifier 16 and thecomponents of the actuation circuit 17) and the most sensitivecomponents are cooled using the cool cooling fluid. Afterwards thecooling channels 39 extend into the external elements of the I-beam 25and into the web of the I-beam 25 where the windings of theheavy-current transformer 12 are cooled, wherein both cooling channels39 streaming in at the side converge into one single cooling channel 39in the web. Then the cooling channels 39 end in a common outlet 33 forthe cooling fluid. The cooling channels in the contact plates 29 and inthe I-beam 25 are preferably formed by corresponding bore holes 40which, on the corresponding positions, are closed by the closure members41. Between the I-beam 25 and the contact plates 29 correspondingsealing members 42, for example O-rings, for sealing the coolingchannels 39 are arranged (see FIG. 8).

In FIG. 8 the I-beam 25 of the heavy-current transformer 12 is shownseparated from the other components of the heavy-current transformer 12or the power source 10. On the end positions of the cooling channels 39the above-mentioned sealing members 42, for example in the form ofO-rings, are arranged. The recesses 25 a in the I-beam 25 are designedto precisely receive the ring core 15, whereby a very compact design isachieved. Simultaneously, the web of the I-beam 25 forms the contactsurface for the center tapping of the secondary windings 14 of theheavy-current transformer 12. The center tappings of the secondarywindings 14 are connected to the web of the I-beam 25 without lines,whereby in turn corresponding lines can be omitted. By directlyconnecting the secondary windings 14 to the I-beam 25 a substantialincrease of the connection surface is also achieved, and, thus, contactlosses and line losses can again be avoided.

The I-beam 25 forms the basis of the heavy-current transformer 12 aroundwhich the secondary windings 14 are arranged such that connection linesare not required. The outer surfaces of the I-beam 25 represent the twofirst contacts 20, 21 of the power source 10, which are connected to thegun arms 5 of the resistance welding device 1 directly, i.e. withoutlines. A space-saving arrangement is achieved in that the ring cores 15are not designed circular, but oval or flat. Preferably, closed ringcores 15 are employed. By using said design the series/parallelconnection of the primary windings 13 and the secondary windings 14 canbe implemented by which the required transmission ratio of theheavy-current transformer 12 for the heavy direct current to be providedwith reduced numbers of turns of the primary windings 13 and thesecondary windings 14 is achieved. Such a design is especially usefulwhen at least three secondary windings 14 connected in parallel arearranged on each side of the I-beam 25.

FIG. 9 shows the sectional view of the I-beam 25 of FIG. 8 alongintersecting line IX-IX. In this figure the course of the coolingchannels 39 to the common outlet 33 for the cooling fluid can clearly beseen.

FIG. 10 shows a contact plate 29 of the heavy-current transformer 12 orthe power source 10 as well as the circuit board 35 for the synchronousrectifier 16 and the actuation circuit 17 arranged above it in enlargedillustration. As already mentioned above, the circuit elements 24 of thesynchronous rectifier 16 are on one side directly contacted to thecorresponding secondary windings 14 of the heavy-current transformer 12and are, on the other side, directly connected to the contact plate 29.For this purpose protrusions 36, in particular pinnacle-shapedprotrusions, are arranged on the inner surface of the contact plate 29,which protrude into corresponding openings 37 on the circuit board 35and there contact the source connections of the circuit elements 24,which are arranged above the openings 37, directly or without lines.Because of the protrusions 36 connection lines between the circuitelements 24 of the synchronous rectifier 16 and the contact plates 29can be omitted, whereby, one the one hand, ohmic losses can be reducedand, on the other hand, the thermal transfer between the circuitelements 24 and the contact plates 29 can be improved. Finally, theproduction effort is also reduced, since no connection lines need to belaid and connected, but the circuit elements 24 are connected,preferably soldered, directly to the protrusions 36. Also a simplepositioning of the circuit board 35 is enabled and, thus, the productionis substantially simplified.

By arranging the actuation circuit 17 and the synchronous rectifier 16on the circuit board 35, which is arranged on the inner side of thecontact plate 29, the direct contacting or contacting of the connectionsof the secondary windings 14 to circuit elements 24 of the synchronousrectifier 16 without lines is achieved as well as a direct contacting orcontacting of the outputs of the synchronous rectifier 16 to the contactplate 29 without lines. Preferably, the heavy-current transformer 12 orthe power source 10 in order to provide the direct current is designedsymmetrically, wherein on both sides of the secondary windings 14, whichare symmetrically arranged, one circuit board 35 each is arranged with aportion of the synchronous rectifier 16 and the actuation circuit 17below one contact plate 29 each.

In the synchronous rectifier 16 according to FIG. 10 ten circuitelements 24 are each arranged in a row. To ensure that all circuitelements 24 connected in parallel are actuated substantiallysimultaneously and run-time losses have only little effects, asymmetrical actuation of the circuit elements 24 from both sides isperformed, that means that preferably five circuit elements 24 are eachactuated from the right and the left via gate drivers arranged on bothsides. Also different actuation options, such as an additional gatedriver extending centrally, can be arranged, whereby the line lengthsand the inductances thereof are divided into three. By such a parallelactuation of the gates of the circuit elements 24 of the synchronousrectifier 16 short actuation paths and, thus, nearly synchronousswitching times of the circuit elements 24 are ensured, since no orlittle run-time losses occur.

During mounting of the circuit board 35 onto the contact plate 29 theprotrusions 36 of the contact plate 29 protrude through the openings 37of the circuit board 35, whereby the back of the circuit board 35 cansimultaneously be securely connected or soldered to the contact plate29, and in addition circuit elements 24 arranged on the opposite sidemay also be connected or soldered to the contact plate 29. Thus, theusual high amount of wiring can be omitted. Also, an easy positioning ofthe circuit board 35 on the contact plate 29 is possible and it can nolonger slip when soldering. When the synchronous rectifier 16, theactuation circuit 17 and the supply circuit 48 are arranged on thecircuit board 35, an autonomous design can be achieved in theintegration of the circuit board 35 in the heavy-current transformer 12.It is a further advantage to have the actuation circuit 17 arranged onboth sides of the circuit elements 24 which are arranged in parallel andin series, since a shortening of the line ways to the individual circuitelements 24 is achieved. Thus, it can be ensured that within a veryshort period of time all of the circuit elements 24 connected inparallel are switched on. With said both-sided arrangement of theactuation circuit 17 a halving of the line length and, consequently, areduction of line inductances and, thus, a significant shortening of theswitching times is achieved. On one side of the circuit board 35 asolderable face is provided preferably over the entire face in order tobe soldered to the contact plate 29, whereby a secure connection to thecontact plate 29 can be achieved. Thus, the contact resistances can alsobe significantly reduced, because a full-faced connection of the circuitboard 35 has a lower contact resistance. Instead of the preferred directconnecting by soldering, short connection wires, so-called bondingwires, can also be provided.

The supply circuit 48 is preferably designed to form correspondinglyheavy switching currents, for example between 800 A and 1500 A, inparticular 1000 A, and to supply the components with a correspondingsupply voltage. Due to the very heavy switching current a very shortswitching time, especially in the ns range, can be achieved. Thereby itcan be ensured that the circuit elements 24 are always switched at zerocrossing or immediately just before zero-crossing at a low outputcurrent, so that no or hardly any switching losses occur. If a datacommunication circuit for the wireless transmission of data, preferablyinductive, magnetic, or via Bluetooth, is provided, data can wirelesslybe transferred from and to the circuit board 35 (not shown). Thus, anadjustment of the switching time points can be made in different areasof application of the heavy-current transformer 12. Likewise, data canbe read from a storage (not shown) arranged on the circuit board 35 inorder to be further processed or for control or quality control,respectively.

To provide overvoltage protection for the circuit elements 24 of thesynchronous rectifier 16 it is advantageous to switch on the circuitelements 24 when they are not required. This means that in case of theapplication in a resistance welding device 1 the active synchronousrectifier 16 is activated in welding breaks, in order to avoid thedamage of the circuit elements 24. It is monitored whether a primarycurrent or a secondary current is flowing through the heavy-currenttransformer 12, and in case of no current flowing while the welding gun4 is correspondingly positioned for a new welding spot, the actuationcircuit 17 activates all circuit elements 24 by corresponding actuationof the gates. When the power source 10 is activated after positioning ofthe welding gun 4, meaning that a manual or automatic welding process isstarted, an alternating voltage is applied to the primary winding 13 ofthe heavy-current transformer 12, which in turn is detected by theactuation circuit 17 due to a current flowing, and, thus, the protectivemode of the circuit elements 24 is deactivated. Of course, theactivation and deactivation of the circuit elements 24 of thesynchronous rectifier 16 can also be performed via control signals whichare sent to the actuation circuit 17 via radio or inductively ormagnetically. Possible overvoltages can do no harm to the circuitelements 24 switched on. Also a certain minimum protection of thecircuit elements 24 by means of Zener diodes can be provided.

FIG. 11 shows a sectional view of the contact plate 29 according to FIG.10 along intersecting line XI-XI. In this figure the course of thecooling channels 39 can clearly be seen. The openings in the bore holes40, resulting from the manufacturing process, in order to form thecooling channels 39 are sealed by corresponding closure members 41. Theclosure members 41 can be implemented by appropriate screws which arescrewed into corresponding threads within the bore holes 40.

FIG. 12 shows a ring core 15 having two secondary windings 14 of theheavy-current transformer 12 arranged thereon plus a current transducer18 arranged above which is shown in an exploded illustration. Thecurrent transducer 18 is protected from external magnetic fields by theshielding housing 34 and a shielding 43, so that the secondary-sidecurrent through the secondary winding 14 can be measured as precisely aspossible and can be supplied to the actuation circuit 17 in order tocontrol the circuit elements 24 of the synchronous rectifier 16. Forshielding from magnetic fields ferrites are especially suited materials.The current transducer 18 is positioned or secured over a portion of oneof the two secondary windings 14 arranged. As known from prior art thecurrent transducer 18 is formed of a magnetic core with a windingarranged thereon, wherein the contacts of the winding are connected tothe actuation circuit 17. Further, between the ring core 15 and thesecondary winding 14 the shielding 43 as well as a core plate for thecurrent transducer 18 are arranged, wherein the core of the currenttransducer 18 is placed on said core plate.

In this design of the heavy-current transformer 12 two secondarywindings 14 of such a design are arranged on both sides of the I-beam 25so that the actuation circuit 17 measures the current flow through oneof the secondary windings 14 connected and positioned in parallel onboth sides. When the actuation circuit 17 is connected to these currenttransducer 18 a precise control or regulation becomes possible, sincethe states in the heavy-current transformer 12 can be detected via thecurrent transducers 18.

Due to the parallel connection of the secondary windings 14 describedabove, in every secondary winding 14 the same current is flowing. Thus,the current only needs to be tapped from one secondary winding 14, inorder to make a conclusion with respect to the entire current flow. In aparallel connection of ten secondary windings 14 only one tenth of theentire current flow is measured by the current transducers 18, which iswhy these can be sized substantially smaller. Thus, in turn, a reductionof the overall size of the heavy-current transformer 12 or the powersource 10 is achieved. It is advantageous to have the currenttransducers 18 arranged substantially at an orientation of 90° to thedirection of the direct current, in particular the welding current,since thereby interferences by the magnetic field induced by the directcurrent and, thus, measurement errors can be reduced. Thus, a veryprecise measurement can be performed.

As can be seen in the exploded view according to FIG. 13, the secondarywindings 14 of the heavy-current transformer 12 are preferably formed bytwo metal sheets 44, 45, which are insulated from each other by aninsulating layer 46, for example a paper layer, having a substantiallyS-shaped mirror-inverted course around the cross-section of a ring core15 and through the ring core 15, which are arranged in each other. Thatmeans that on one ring core 15 two secondary windings 14 or the parts ofthe secondary winding 14 with center tapping are arranged. The exteriorsurfaces 47 of the metal sheets 44, 45 of the secondary windings 14simultaneously form the contact surfaces for the contacting to thecircuit elements 24 of the synchronous rectifier 16 and the I-beam 25acting as a center of the rectification. Thus, no lines are required inorder to connect the secondary windings 14 of the heavy-currenttransformer 12 to the circuit elements 24 of the synchronous rectifier.The secondary windings 14, especially the metal sheets 44, 45 formingthe secondary windings 14, are connected to the circuit elements 24 ofthe synchronous rectifier 16 and to the web of the I-beam 25 or thecenter of the rectification directly or without lines. Thus, a veryspace-saving and compact, lightweight design with low losses isachieved. Simultaneously relatively large surfaces 47 for connecting thesecondary winding 14 to the web of the I-beam 25 and the circuitelements 24 of the synchronous rectifier 16 are provided for contacting,in order to ensure the heavy current flow with as little losses aspossible. By such an arrangement a center rectifier is implemented onthe secondary side, wherein the I-beam 25 is forming the center with theone connected end of the secondary windings 14.

The ring core 15 can be made of ferrites, amorphous materials ornanocrystalline raw materials. The better the materials used are withregard to the magnetic properties, the smaller the ring core 15 can bedesigned. However, the price of the ring core 15 is, of course, rising.In designing the metal sheets 44, 45 it is substantial that they will befolded or bent in a manner such that they are passed through the ringcore 15 at least once. The two metal sheets 44, 45 or secondary windings14 arranged on one ring core 15 are designed in a mirror-inverted wayand are insulated from each other.

FIG. 14 shows a block diagram of a supply circuit 48, especially a powersupply unit, in order to supply the synchronous rectifier 16 and theactuation circuit 17 with electric energy. The supply circuit 48 isconnected to the secondary side or the connections of the secondarywinding 14 of the heavy-current transformer 12 and includes a peak valuerectifier 49, a voltage increaser 50, a linear voltage regulator 51 anda voltage divider 52. The voltage increaser 50 or booster ensures thatthe supply of the components of the power source 10 is provided quickly.Simultaneously the internal supply voltage of the active synchronousrectifier 16 is generated as quickly as possible. By using the voltageincreaser 50 it is ensured in the initial phase of activation that therequired amplitude of the supply voltage is generated at first as earlyas possible, in order to ensure a secure function of the synchronousrectifier 16 integrated into the heavy-current transformer 12 at a timeas early as possible.

FIG. 15 shows a time course of the supply voltage V of the supplycircuit 48 according to FIG. 14. The ramp of the voltage increase ΔV/Δtis selected sufficiently steep to ensure that the required voltage VCCis applied with a maximum delay T_(d) at the synchronous rectifier 16and the actuation circuit 17. For example, the delay T_(d) should be<200 μs. By appropriate configuration of the circuits of the peak valuerectifier 49 and the voltage increaser 50 and appropriately lowcapacities a sufficient slew rate of the voltage can be achieved. Thus,it can be said that at first the minimum height of the supply voltage isensured with a steep increase and only then the proper supply iscreated.

FIG. 16 shows time courses of the secondary-side current I_(S) of theheavy-current transformer 12 and of the control signals G₁ and G₂ forthe circuit elements 24 of the synchronous rectifier 16 for illustrationof the loss-free actuation. By measuring the secondary-side currentsI_(S) of a secondary winding 14 using corresponding current transducers18 the actuation circuit 17 obtains information, at which points in timethe circuit elements 24 of the synchronous rectifier 16 should beswitched. To reduce conduction and switching losses the circuit elements24 of the synchronous rectifier 16 are preferably switched in zerocrossing of the secondary-side currents through the secondary windings14 of the heavy-current transformer 12. Since certain delays t_(Pre)occur from the detection of the zero crossing of the secondary-sidecurrent I_(S) by the current transducers 18 to the activation of thecircuit elements 24 of the synchronous rectifier 16, the actuationcircuit 17 is, according to the invention, formed to actuate the circuitelements 24 of the synchronous rectifier 16 at a preset time prior toreaching zero crossing of the current in the secondary winding 14. Thus,the actuation circuit 17 causes the switching the of circuit elements 24of the synchronous rectifier 16 at points in time at which the currentsI_(S) in the secondary winding 14 of the heavy-current transformer 12measured by the current transducers 18 fall below or exceed a certainswitch on threshold I_(SE) and switch off threshold I_(SA). By usingthis method it can be achieved that the circuit elements 24 of thesynchronous rectifier 16 are switched substantially during zero crossingof the currents I_(S) through the secondary windings 14 of theheavy-current transformer 12, whereby the conduction losses andswitching losses of the circuit elements 24 of the synchronous rectifier16 can be minimized. The switch on and switch off times of the circuitelements 24 of the synchronous rectifier 16 are, thus, not determined bythe zero crossing of the secondary-side current, but by the achievementof the defined switch on threshold I_(SE) and switch off thresholdI_(SA). The switch on threshold I_(SE) and the switch off thresholdI_(SA) are defined according to the switching delays to be expected. Theswitch on threshold I_(SE) and the switch off threshold I_(SA) are atbest designed adjustable, in order to further reduce the losses. In aheavy-current transformer 12 of 20 kA the switching time can for examplebe set 100 ns prior to zero crossing so that the circuit elements 24 ofthe synchronous rectifier 16 need to be switched within this timeperiod.

A common prior art heavy-current transformer for a resistance weldingdevice in order to provide a welding current of for example 20 kAexhibits losses of approximately 40-50 kW. To provide a welding currentof 20 kA according to prior art a connected wattage of up to 150 kW isrequired in total, wherein the total losses add up to approximately 135kW resulting in an efficiency of some 10%. In contrast, a heavy-currenttransformer 12 of the present invention exhibits losses of only some 5-6kW. Line losses can be lowered from usually 30 kW to 20 kW. Thus, in aresistance welding device 1 according to the invention the connectedwattage for generating a welding current of 20 kA can be reduced to 75kW, since total losses add up to some 60 kW only. Hence, with some 20%the resulting efficiency is approximately twice as high as in prior art.From this comparison potential savings can be seen clearly, inparticular in production lines in the automotive industry comprising aplurality of resistance welding devices.

Basically the power source 10 or the heavy-current transformer 12described is designed in the form of a cube or ashlar, wherein two sidesurfaces are formed by an I-beam 25, on which side surfaces contactplates 29, which are electrically insulated for forming the third andfourth side surfaces, are arranged. At the front face a cover plate 31is each arranged towards the four side surfaces, which is electricallyinsulated against the I-beam 25, in order to form the fifth and sixthside surfaces of the cube or ashlar. Within the cube, in particularwithin the side surfaces, the synchronous rectifier 16 and the actuationcircuit 17 are arranged on at least one circuit board 35 or printedcircuit board. Thus, the cube only has connections 26 for the primarywindings 13 of the heavy-current transformer 12 and the side surfaces ascontact surfaces for consumption of the direct current or the directvoltage. In addition, cooling connections, in particular the inlets 32and the outlet 33 for a cooling fluid, are also provided. Control linesfor the synchronous rectifier 16 and the actuation circuit 17 integratedin the cube are preferably not provided, since this system operatesautonomously and, thus, connections to the power unit 19 or to a controldevice of the system are not necessary. In such a design preferably nocontrol lines are required, but the power source 10 is connected to apower unit 19 on the primary side only, whereupon on the secondary sidethe correspondingly dimensioned direct current of for example 15 kA to40 kA is available. Thus, the user is not required to make anyadjustments, but only needs to connect the power source 10. Theintegration of the actually independent separate components into such acommon unit causes a substantial reduction in the overall size and,thus, the weight of the power source 10. Simultaneously, the unit canalso be employed as a supporting element directly in an application, inparticular a welding gun 4. Also the user convenience is substantiallyincreased.

In the present design it is further important to connect the circuitelements 24 to the corresponding components without lines, i.e., thesource connections of the circuit elements 24 formed by field-effecttransistors conducting the welding wire are directly connected orsoldered to the protrusions 36 of the contact plate 29, wherein the gateconnections of the circuit elements 24 are also arranged or soldereddirectly to the circuit board 35 and to the actuation circuit 17 (gatedriver) built thereupon. Thus, line inductances can be reduced by fullyomitting the lines so that high switching speeds and very low conductionlosses can be achieved.

In the embodiment shown and described, the heavy-current transformer 12was dimensioned for a current of 20 kA at an output voltage of between5V and 10V. The I-beam 25 has an overall height of 15 cm so that on bothsides five secondary windings 14 having ring cores 15 can each bearranged. To get a corresponding transmission ratio of 100, ten primarywindings 13 are required in the embodiment shown.

When the heavy-current transformer 12 is now to be dimensioned for amore heavy current of for example 30 kA, the number of secondarywindings 14 used can simply be increased. For example, in the recesses25 a on both sides of the I-beam 25 seven secondary windings 14 can eachbe arranged, wherein the I-beam 25 in its height is correspondinglyenlarged, for example designed by only 5 cm higher or a correspondinglylarger base body is employed. Thus, the I-beam 25 of the heavy-currenttransformer 12 is supplemented on both sides by two secondary windings14 only, in order to be able to provide a more heavy current. By saidenlargement the contact cooling surfaces are also enlarged. Further,correspondingly more circuit elements 24 will be arranged in parallel.The primary winding 13 can be reduced to a lower number of turns, forexample seven turns, so that a transmission of for example 98 isachieved. Due to the possible increase of the cross-section and byreduction of the line length higher primary winding losses arecompensated for by the heavier primary current.

Thus, an increase of the secondary welding current from 20 kA to 30 kAonly results in an elongation of the cube or heavy-current transformer12 by for example 5 cm.

Since the heavy-current transformer 12 preferably operates autonomouslyand comprises no control lines, an outgoing communication with externalcomponents, in particular a control device, for possible error messagesshould be enabled. For this purpose the secondary circuit consisting ofthe secondary windings 14 and the synchronous rectifier 16 and theactuation circuit 17 can be used. In certain states, in particular inthe idle state of the heavy-current transformer 12, said heavy-currenttransformer 12 can be consciously short-circuited using the synchronousrectifier 16 so that an idle state current flow in the primary lines canbe detected by an external monitoring unit or a control device and,thus, due to the current a communication or error message can beeffected.

For example, by integrating a temperature sensor in the heavy-currenttransformer 12, in particular on the synchronous rectifier 16, thetemperature can be detected and evaluated. If the temperature forexample exceeds a defined threshold, the synchronous rectifier 16 in theidle state, that is during welding breaks, is definedly short-circuitedby the actuation circuit 17. Since the external control device knows theidle state in which no welding is performed, it is detected orrecognized by the increased current flow in the primary lines of theheavy-current transformer 12. Now it can be examined by the externalcontrol device whether the cooling circuit is activated or shows adefect or the cooling efficiency is increased so that a better coolingis performed.

Of course, via corresponding switching or pulse patterns, i.e. definedopening and closing of the circuit elements 24 of the synchronousrectifier 16 in the idle state, different error messages can becommunicated outwardly. For example, different temperature values,secondary voltages, currents, error messages, etc. can be sent outwards.

However, it is also possible that such a communication is performedduring a welding, although such a detection is clearly more difficult.For example, corresponding signals can be modulated onto theprimary-side current, in particular by the primary windings 13.

The invention claimed is:
 1. A resistance welding device comprising: acube or ashlar-shaped power source in order to provide a welding currentfor welding workpieces, a welding gun with a first gun arm and a secondgun arm, each of the first gun arm and the second gun arm comprising arespective electrode in order to impress the welding current into theworkpieces, wherein the power source comprises a heavy-currenttransformer with at least one primary winding and at least one secondarywinding with center tapping, a synchronous rectifier connected with theat least one secondary winding of the heavy-current transformer andcomprising circuit elements, and an actuation circuit for actuating thecircuit elements of the synchronous rectifier, wherein the power sourceis arranged at the welding gun and the power source comprises a firstcontact, a second contact, a third contact and a fourth contact forminga multi-point contacting at an output of the synchronous rectifier,whereby the welding current is at least halved and thus the transitionlosses are reduced, wherein the first contact and the second contact areof one polarity and are connected to the first gun arm and the thirdcontact and the fourth contact are of opposite polarity and areconnected to the second gun arm, wherein the first contact and thesecond contact of the one polarity and the third contact and the fourthcontact of opposite polarity are each arranged opposite each other,wherein the third contact and the fourth contact are arranged offset toeach other by 90° compared to the first contact and the second contact,such that the first contact, the second contact, the third contact andthe fourth contact form the cube or ashlar-shaped power source, whereinthe first gun arm is fastened without lines to the first contact and thesecond contact, wherein the heavy-current transformer comprises anI-beam of electrically conductive material, the I-beam having aplurality of outer surfaces and a plurality of recesses in whichrecesses at least one respective ring core is arranged, wherein onerespective connection of the at least one secondary winding is directlycontacted with one inner surface of the I-beam, wherein the outersurfaces of the I-beam form the first contact and the second contact ofthe power source, and wherein the center tapping of the at least onesecondary winding of the heavy-current transformer is connected with theI-beam without lines.
 2. The resistance welding device according toclaim 1, wherein the synchronous rectifier and the actuation circuit areintegrated in the heavy-current transformer.
 3. The resistance weldingdevice according to claim 1, wherein the circuit elements are connectedin parallel and wherein the circuit elements are connected without linesto the at least one secondary winding of the heavy-current transformer.4. The resistance welding device according to claim 1, wherein the atleast one primary winding comprises a plurality of primary windingsconnected in series, and the at least one secondary winding comprises aplurality of secondary windings with the center tapping connected inparallel.
 5. The resistance welding device according to claim 1, whereinthe at least one primary winding of the heavy-current transformer isarranged to run through the at least one respective ring core.
 6. Theresistance welding device according to claim 1, wherein contact plateseach having a plurality of outer surfaces, are located above therecesses of the I-beam, said contact plates being of electricallyconductive material and connected with the respectively otherconnections of the at least one secondary winding via the synchronousrectifier and the actuation circuit, wherein the outer surfaces of thecontact plates form the third contact and the fourth contact of thepower source.
 7. The resistance welding device according to claim 6,wherein the I-beam and the contact plates of the heavy-currenttransformer form a cube or ashlar-shaped unit, wherein an electricalinsulation is arranged between the I-beam and the contact plates,wherein cover plates are arranged at front faces of the I-beam, andwherein the cover plates are formed of electrically conductive materialand are adapted to be screwed to the contact plates, so that the contactplates are connected electrically.
 8. The resistance welding deviceaccording to claim 6, wherein a plurality of cooling channels forfeeding a cooling fluid are arranged in the I-beam and in the contactplates, wherein two inlets for feeding the cooling fluid and one outletfor discharging the cooling fluid are arranged at an outer surface ofthe I-beam, wherein the cooling channels are arranged to extend fromeach inlet to the contact plates and via the I-beam to the outlet. 9.The resistance welding device according to claim 6, wherein theactuation circuit and the synchronous rectifier are arranged on at leastone circuit board which is arranged on an inner surface of at least onecontact plate, and wherein each circuit board of the synchronousrectifier and of the actuation circuit comprises openings above whichthe circuit elements are arranged, and wherein the inner surface of thecontact plates, at locations corresponding to the openings in thecircuit board of the synchronous rectifier, comprises protrusions, sothat the circuit elements are contacted without lines via theprotrusions protruding into the openings on the circuit board on theinner surface of the contact plates.
 10. The resistance welding deviceaccording to claim 1, wherein the at least one secondary winding withthe center tapping is formed by two metal sheets having respectiveexterior surfaces, the two metal sheets being insulated from each otherand being of electrically conductive material and having an S-shapedmirror-inverted course around a cross-section of a ring core and throughthe ring core, wherein the exterior surfaces of the metal sheets formthe first contact, the second contact, the third contact and the fourthcontact for the connection with the circuit elements of the synchronousrectifier or the respective electrode.
 11. The resistance welding deviceaccording to claim 1, wherein at least one of the I-beam, a plurality ofcontact plates, a plurality of cover plates and a plurality of metalsheets for forming the at least one secondary winding of theheavy-current transformer are formed of copper or a copper alloy. 12.The resistance welding device according to claim 1, wherein at least oneof the I-beam and a plurality of contact plates are formed at leastpartially by the first gun arm and the second gun arm.
 13. Theresistance welding device according to claim 1, wherein a currenttransducer is arranged at a connection of the at least one secondarywinding for measuring a current through the at least one secondarywinding, said current transducer being connected with the actuationcircuit.
 14. The resistance welding device according to claim 13,wherein the current transducer is arranged to be oriented 90° to adirection of the welding current.
 15. The resistance welding deviceaccording to claim 13, wherein the actuation circuit is designed toactivate the circuit elements of the synchronous rectifier at a presetpoint in time prior to achieving zero crossing of the current in the atleast one secondary winding.